Roosters Only ?

I have had 4 roosters die in the last month,all 16 hens doing just fine. All of the roosters were 2-4 years old, all were doing just fine at their evening check-in, dead on the coop floor in the morning, each rooster had it's own coop with his own girls. No mites or bugs of any kind, no injuries. All of my birds free range during the day, cooped up at night. The roosters were all different breeds and not related.

I am at a loss for an explanation, anyone have any idea what it could be ?